Generally, well treatments involve the injection of a fluid into an oil or gas formation to stimulate production from the well by increasing the permeability of the oil or gas through the formation.
A widely used stimulation technique is acidizing, in which an aqueous acid treatment is introduced into the formation to dissolve acid-soluble materials that clog or constrict formation channels. In this way, hydrocarbon fluids can more easily flow from the formation into the well. Also, acid treatments facilitate the flow of injected treatment fluids from the well into the formation.
Another common stimulation technique is hydraulic fracturing, in which a fracturing fluid is injected through a well into the surrounding formation at a sufficient pressure to fracture the formation adjacent to the well, creating a channel for fluid flow through the formation back to the well. Usually a particulate material, often referred to as a “proppant,” is deposited into the fracture to help prop the fracture open for fluid flow back after the hydraulic pressure is released.
Acidizing techniques can be carried out as “matrix acidizing” procedures or as “acid fracturing” procedures.
In matrix acidizing, the acidizing treatment fluid is injected from the well into the formation at a rate and pressure below the pressure sufficient to create a fracture in the formation. The acid permeates into channels and dissolves materials that clog or constrict the channels, thereby increasing permeability of the formation. Thus, an increase in permeability is affected primarily by the reaction of the acid within the formation, and little or no permeability increase is due to induced fractures within the formation.
In acid fracturing, an increase in permeability is affected by fractures as well as by the acid etching through the channels within the formation. The acidizing treatment fluid is injected into the well that is disposed within the formation to be fractured. Sufficient pressure is applied to the acidizing treatment fluid to cause production of one or more fractures in the formation.
While hydrocarbon producing wells are usually completed in hydrocarbon-producing formations, the formations frequently contain layers of water-bearing sections or may be located adjacent to water-bearing sections. The high mobility of the water often allows it to flow into the wellbore by way of natural fractures and/or high permeability streaks present in the formation. Over the life of such wells, the ratio of water to hydrocarbons recovered often becomes so high that the cost of producing the water, separating it from the hydrocarbons, and disposing of the waste water represents a significant economic loss.
Furthermore, when an acidizing treatment fluid is required to increase the productivity of a hydrocarbon-bearing interval, the aqueous fluid tends to predominately enter a water-bearing section instead of a hydrocarbon-bearing section. This is because the water-bearing section is relatively more permeable to the aqueous fluid than the hydrocarbon-bearing section. Thus, acid stimulation often results in increasing the water cut because of the preferential stimulation of the water-bearing section.
The production of water with hydrocarbons, i.e., oil and/or gas, from wells constitutes a major problem and expense in the production of hydrocarbons from subterranean formations. The expense includes the energy in moving the water to the surface, separating the water from the produced hydrocarbon, and disposing of the waste water.
A variety of techniques to divert the aqueous acidizing treatment fluid away from a water-bearing section and into a hydrocarbon-bearing section have been attempted. By injecting particulates, foams, or blocking polymers prior to or along with acidizing treatments, the water-bearing section is attempted to be plugged off. In this way, the acid treatment can predominantly enter and stimulate the hydrocarbon-bearing section rather than the water-bearing section.
While the use of these water-blocking techniques has achieved varying degrees of success, there are many challenges in their use. For example, the blocking polymers are injected into the formation and cross-linked to form stiff gels capable of stopping or reducing the flow of the undesired water. Even when a polymer solution is properly placed in a water-producing zone, however, the cross-linked gels formed often do not remain stable in the zone due to thermal degradation and/or differences in the adsorption characteristics of the polymer and associated cross-linker and the like.
Furthermore, techniques geared toward injecting materials designed to plug off the water-bearing section are limited because many require expensive zonal isolation. Also, zonal isolation is sometimes inaccurate, which may lead to inadvertently plugging and damaging the hydrocarbon-bearing section. Damage to hydrocarbon-producing pathways is undesirable since it reduces well productivity and profitability. The desired end result is to reduce the effective permeability to water in the treated portion of the zone without loss of effective permeability to hydrocarbons.
Recently, chemicals have been utilized to decrease the production of water with hydrocarbons. These chemicals are referred to as relative permeability modifiers (“RPMs”), sometimes referred to as disproportionate permeability reducers or selective plugging systems. An RPM polymer such as polyacrylamide is dissolved in water and pumped into a subterranean formation that produces water and hydrocarbon, reducing the permeability of water through the formation without substantially affecting the permeability of hydrocarbon. That is, water permeability modifying chemicals such as polyacrylamide have been introduced into hydrocarbon and water producing formations so that the chemicals attach to adsorption sites on surfaces within the porosity of the formations.
The presence of the RPM chemicals in the formations has the effect of reducing the flow of water through the formations. The purpose of water permeability modifying chemicals in hydrocarbon and water producing formations to decrease the production of water involves less risk than other techniques such as blocking the flow of water with cross-linked polymers, and has the advantage that they do not require expensive zonal isolation techniques.
The use of such conventional water permeability modifying chemicals, e.g., polyacrylamides, however, has heretofore resulted in only small temporary reductions in water production and/or unacceptable levels of reduction in hydrocarbon production. Conventional RPM polymers have provided poor performance due to inadequate retention within the formation. Many of these conventional RPM water-control compounds are unstable in acids and heavy brines and/or they may degrade with increased temperature, rendering them useless in many downhole applications.
Thus, there is a need for improved methods of treating subterranean formations to direct the acidizing treatment fluid away from the water-bearing section and into sections capable of producing hydrocarbon, while maintaining stability of such treatment fluid in the downhole acidizing environment.